There were reports that the agency was hit with the Neeris worm, which infects desktops and can enable a remote user to execute malicious commands on the affected system.

Neeris and its variants are capable of propagating using multiple avenues including network shares and removable drives, via software vulnerabilities in servers to propagate across networks, and via Microsoft's instant messaging clients.Trend Micro lists the risk rating for Neeris as "Low" but the damage potential as "High."

Michael Sweeny, global public relations director for Trend Micro, said the U.S. Marshals Service had contacted his company last night for help with its network issues.

He did not detail what those problems were and said he had not heard anything about Neeris being the culprit.

But Trend Micro's daily statistics on Neeris worm infections showed a spike from 7 p.m. to 8 p.m. May 20 that rose from nearly zero to 700 computers. Another smaller spike of about 100 computers was detected from 6 a.m. to 7 a.m. this morning (May 21).